Nanoscale Insights Into Hydration‐ and Light‐Induced Surface Degradation and Annealing Recovery in Solution‐Processed Perovskite Thin Films

Using the same‐location atomic force microscopy (AFM), we investigated how hydration, thermal annealing, and optical irradiation affect the nanoscale surface evolution of solution‐processed formamidinium–methylammonium lead iodide (FA _0.6 MA _0.4 PbI ₃ ) thin films. Upon exposure to high relative humidity (∼85% RH) at room temperature, the polycrystalline films exhibited two distinct nanoscale degradation pathways: the formation of depressions along grain boundaries and the development of pits at pre‐existing defect sites within grain interiors. To explain these observations, we propose a defect‐initiated hydration degradation mechanism, supported by direct nanoscale morphological evidence for surface‐initiated degradation at both grain‐boundary and grain‐interior sites. Thermal annealing partially recovers these hydration‐induced surface degradations, reducing the depth of pits and depressions, narrowing the grain boundary features, and sharpening the step edges, indicating partial morphological recovery. Furthermore, UV‐A light and 405 nm laser irradiation induce real‐time surface roughening and step‐edge broadening, accompanied by apparent surface depletion. We discuss these observations within the framework of established degradation mechanisms. Our same‐location AFM tracking provides nanoscale, time‐resolved insights into the degradation behavior of the FA _0.6 MA _0.4 PbI ₃ thin films that can guide the design of more durable perovskite devices for real‐world environments.